Various formulations of thermoplastic compositions have been developed in an attempt to at least partially replace non-degradable petroleum-based products with biodegradable components which can be used for the manufacture of extruded and/or molded articles such as films, utensils, containers and other packaging articles. Several of these compositions have been formulated with starches. Although starch compositions are biodegradable, they also tend to readily absorb moisture from humid air or by direct contact with water. This causes the moisture content of a molded article to increase and the article to lose its dimensional stability, and tear or collapse.
In an attempt to improve the structural stability of articles made from starch-based compositions, other ingredients have been included in the formulations. For example, compositions have been developed that include starch in combination with a water-insoluble synthetic polymers. Unmodified starches have also been combined with protein to provide moldable, biodegradable thermoplastic compositions. For example, Nakatsuka et al. (U.S. Pat. No. 4,076,846; issued Feb. 28, 1978) discloses an edible binary protein-starch molding composition containing a salt of a natural protein (i.e., casein), an unmodified, high amylose starch material, an edible plasticizer (i.e., sorbitol), and a lubricant (i.e., a fatty acid polyol ester), and having a final water content of about 10-40%. The composition is molded, for example, by extrusion through a die, into an article having a water content of about 5-30 wt-%. A disadvantage of these starch-based plastics is that the molded articles made from such compositions have a high tendency to absorb water, which causes the articles to lose mechanical strength and to disintegrate quickly.
Therefore, an object of the invention is to provide a thermoplastic material made of starch and protein which can be formed into articles that have an increased level of mechanical strength and water-resistance, being capable of tolerating exposure to water over an extended period of time with minimal or no disintegration. It is a further object to provide a thermoplastic material that is biodegradable.